radeon - How to change the graphics card driver via terminal?

I recently tried changing the driver for my Asus Radeon HD 7870 to the proprietary one (a friend told me it was much faster), however that seems to have killed my install of Ubuntu. Now whenever I boot up it shows me GRUB, I select Ubuntu, and then it boots up for a second or two and then blackscreens. I think the blackscreen is caused by it reaching the login screen, as it is also at that point that I can SSH into it.

So can anyone tell me how to switch back to the open source driver?

Edit: And I'll add, when I boot Ubuntu off of a USB, everything works fine.

Edit: Ok here my process is for the driver.

1. Install Radeon HD 7870 in my system


2. Boot up, everything is working fine automatically


3. I go and check and it is using the open source driver (I checked by going into the Software and Updates program and going to the driver tab)


4. A friend recommends I switch to the propietary one as it is faster (I believe it was something along the lines of flgrx-updates)


5. I go into the Software and Updates program, and navigate to the drivers tab. From there I selected the flgrx-updates one and hit ok.


6. I reboot my system, and then it won't work. After some basic troubleshooting, I come here.

Update: Thanks to gertvdijk I have managed to disable the propietary driver, but Ubuntu still will not beat up. I'm guessing that for whatever reason the open source driver has not been automatically enabled. Anyone have any ideas? Are there any system logs I can give you guys to help find the problem?